1. Field of the Invention
This invention relates to a system for metering flow of molten glass from a glass melter onto a pool of molten metal contained in a forming chamber of a flat glass making apparatus. The metering system includes a glass ribbon edge detector and a device for monitoring tweel movement.
2. Discussion of the Technical Problems and Present Technology
In the manufacture of flat glass, batch materials fed into the upstream end of a melter or furnace melt into raw molten glass as they advance downstream through the melter and, thereafter, the raw molten glass is refined. The refined molten glass exits at the downstream end of the furnace, between a tweel and a threshold or lip onto a pool or bath of molten metal. The tweel is mounted for movement toward and away from the threshold to meter or control the flow of molten glass onto the pool of molten metal. The refined molten glass as it advances downstream on the molten metal pool is controllably cooled to form a glass ribbon which is lifted from the pool by lift out rolls and moved through an annealing lehr.
Factors that are known to control ribbon thickness and width include, among other things, longitudinal and lateral forces acting on the glass ribbon, temperature of the molten glass and flow of the molten glass onto the molten metal. Of particular interest in this discussion is the flow of molten glass onto the molten metal pool. More particularly, assuming that the other factors are constant or stable, decreasing the flow of molten glass decreases the ribbon width and increasing the flow of molten glass increases the ribbon width.
In U.S. Pat. No. 3,500,548, the edge of a glass ribbon is detected by suspending a plurality of spaced electrodes which conduct current when in contact with the molten metal bath at each opposite side of the ribbon. When the ribbon width decreases, ones of the spaced electrodes not supported on the glass contact the molten metal. The resultant variant electrical current is measured to determine the ribbon width. A limitation of the above technique is the electrodes contacting the ribbon surface which could mar the ribbon surface. In U.S. Pat. No. 3,482,954, the ribbon edge is sensed by projecting an annular gas stream toward the ribbon edge and sensing the back pressure. A limitation of this technique is that the air directed onto the molten metal pool may result in turbulence that could have a detrimental affect on the optical quality of the glass.
In U.S. Pat. No. 3,805,072, a glass ribbon edge detector includes a scanning telescope having a pair of cells responsive to infra red energy. The object end of the telescope is mounted above the ribbon edge in a home position as determined by sensed infra red energy. When the ribbon shifts in a direction generally transverse to its direction of motion, the output of the cells varies and the telescopes move in response thereto until the telescopes are each in a home position. The linear displacement and direction of displacement of the telescope at each opposed ribbon edge is sensed to determine the ribbon width. In U.S. Pat. No. 4,008,062, the radiation receiving end of a radiation pyrometer is inserted transversely of and above a glass ribbon supported on a molten metal pool. The radiation receiving edge reciprocably moves until a rapid temperature increase is sensed by the pyrometer to indicate the edge of the ribbon. In U.S. Pat. No. 3,977,858, two optical detectors move in a direction perpendicular to that of a glass ribbon advancing on a roller conveyer. The detector at each side of the ribbon stops upon detecting its respective ribbon edge. The distance between the detectors which corresponds to the width of the glass ribbon is indicated in a form of an electrical signal. A limitation of the ribbon edge detectors taught in U.S. Pat. Nos. 3,805,072, 4,008,062 and 3,977,858 is that the sensing elements are in a water-cooled housing to prevent thermal damage to the sensing elements and the housing. Mounting a water-cooled housing in a heated glass forming chamber necessitates specially designed equipment. Further, condensates form on water-cooled surfaces within the forming chamber and excess condensates may drop onto the ribbon surface causing surface defects. Still further, initial insertion of water-cooled equipment may upset the thermal equilibrium in the forming chamber.
In view of the above, it would be advantageous to provide a ribbon edge detector that provides ribbon width information without having the limitations of the above discussed edge detectors. cl SUMMARY OF THE INVENTION
This invention relates to a method of and apparatus for sensing the edge of a substrate supported on a conductive surface, for example a glass ribbon supported on a molten metal bath. A conductive member positioned over the molten metal bath and the molten metal bath are electrically connected to a power source to provide a capacitor having a voltage output. The voltage output of the capacitor is monitored as the conductive member moves relative to the ribbon edge to detect the ribbon edge. For example, at a minimum voltage output the conductive member is positioned over the molten metal bath and at a maximum voltage output the conductive member is positioned over the glass ribbon. The range in between is a function of the ribbon edge portion between the conductive member and the metal bath.
Further, this invention relates to a method of and apparatus for following the edges of the glass ribbon by positioning a conductive member, e.g. a carbon member on each side of the ribbon to provide a pair of capacitors each having an output voltage. The output voltage of each capacitor is monitored and the conductive probe moved in response to its respective output voltage to follow the ribbon edge. The linear displacement and direction of displacement of each conductive member is used to follow the edge of the ribbon and determine the ribbon width.
Still further, this invention relates to a method of and apparatus for metering the flow rate of molten glass from a glass melter onto a pool of molten metal contained in the forming chamber. A conductive member is mounted over the molten metal bath at each side of the ribbon and connected with the molten bath to provide a pair of opposed capacitors each having a voltage output. The voltage output of each capacitor is monitored and the conductive member moved in response thereto to maintain the conductive member over the edge of the ribbon. The linear displacement and direction of displacement of each of the conductive members determine the change in ribbon width. The change in ribbon width information is analyzed to determine the direction of tweel displacement relative to the threshold or lip for increasing or decreasing the flow rate of molten glass. For example, as the ribbon width decreases, the tweel is moved away from the threshold to increase the flow rate of molten glass and as the ribbon width increases, the tweel is moved toward the threshold to decrease the flow rate of molten glass.
This invention still further relates to a tweel monitoring device for metering glass onto the molten metal bath. The device includes facilities for moving the tweel along a reciprocating path toward and away from the threshold or lip and facilities mounted on each side of the tweel and responsive to the movement of the tweel for generating a signal as a function of tweel displacement and direction along the path. For example, a linear displacement gauge mounted on each side of the tweel has its extending reciprocating rod or member connected to the tweel for direct response to the movement of the tweel. The output voltage of the gauges is acted on to determine tweel displacement and direction of displacement.
The instant invention eliminates the drawbacks of the present edge detectors by employing a conductive member, for example a carbon member which is capable of withstanding the high temperature and hostile environment of the forming chamber of a flat glass making apparatus without the need for fluid cooling equipment. By eliminating fluid cooling equipment, thermal equilibrium within the chamber is not disturbed when the carbon probe is mounted in the chamber and condensates are minimized if not eliminated because the carbon probe is at about the same temperature as the environment within the furnace. Further, the instant invention provides a tweel displacement monitoring apparatus that is operatively connected directly to the tweel. In this manner, the errors normally associated with measuring tweel displacement through the lifting arrangement is eliminated.